Cell search method for orthogonal frequency division multiplexing based cellular communication system

ABSTRACT

A method for synchronizing and identifying the cell code (scrambling code) of a cell in an orthogonal frequency division multiplexing (OFDM) based cellular communication system is provided. In this method, desired cell is found by utilizing a frame structure of OFDM symbols and through a corresponding cell search procedure, where the frame structure has periodic signal pattern and contains the information about the cell code of the desired cell in common pilot channel (CPICH) signal. And, the cell search method utilizes the periodic property of the frame structure to detect frame timing, and the correlation property of CPICH signal to identify the cell code. The cell search method of the present invention offers the advantages of good link quality, fast acquisition, and low power consumption.

FIELD OF THE INVENTION

The present invention generally relates to orthogonal frequency divisionmultiplexing (OFDM) based technology such as orthogonal frequencydivision multiple access (OFDMA) technology and combination of OFDM andcode division multiple access (CDMA) technologies, known as OFDM-CDMAtechnology, and more particularly to a cell search method for OFDM basedcellular communication systems.

BACKGROUND OF THE INVENTION

In an OFDM based cellular communication system, a mobile station (MS)may receive downlink-transmitted signals from different cells and mayneed to differentiate these signals by using different cell codes. Forexample, in an OFDM-CDMA cellular system, the downlink-transmittedsignals from different cells are differentiated by using scramblingcodes (cell codes), thereby allowing for reuse of frequency andspreading codes in contiguous cells. As such, an MS terminal, whenswitched on, needs to search for a cell (i.e., synchronizing to theassociated downlink scrambling code) before any communication. Thisprocedure is known as initial cell search. On the other hand, duringactive or idle modes of an MS terminal, searching for a cell is alsoneeded for identifying handoff candidates. This procedure is known astarget cell search. The performance of cell search method directlyimpacts the perceived switch-on delay, link quality and powerconsumption of an MS. Therefore, cell search is important for the designof OFDM based cellular communication systems.

For brevity, cell search methods here are described in terms of onlymulti-carrier CDMA (MC-CDMA) system, one type of OFDM-CDMA systems,although they may also be applied to other OFDM based systems.Conventional cell search methods for MC-CDMA cellular systems includetwo types of methods, synchronization channel (SCH)-based method andcommon pilot channel (CPICH)-based method, in which cell searchprocedure is highly dependent on the frame structure ofdownlink-transmitted signal.

Consider that there are J downlink scrambling codes, denoted byC^((i))[k], k=0˜K−1, i=1˜J, allowing for unique cell identification inevery cluster of J cells where K is the length of the scrambling codes.Assume that cell j with the scrambling code C^((j))[k] is the desiredcell to be searched for. Typically, the J cells are further divided intoseveral groups to reduce the number of scrambling codes to be searchedfor, where each group is represented by a group code.

FIG. 1 shows the frame structure of the SCH-based cell search method.Each frame consists of M OFDM symbols. Each OFDM symbol consists of notonly N_(FFT)-sample useful data but also N_(GI)-sample cyclic prefix(CP), namely, guard interval (GI), for avoiding intersymbol interference(ISI) as well as inter-carrier interference (ICI). Accordingly, thelength of an OFDM symbol is N_(OFDM)=N_(FFT)+N_(GI). Thedownlink-transmitted signal in FIG. 1 includes three types of signals,CPICH signal, SCH signal, and traffic channel (TCH) signal. CPICH signalcontains the information about the scrambling code, while SCH signalabout the group code and frame timing. TCH signal is used fortransmitting TCH data. In the transmitter of the base station (BS) incell j, the data of TCHs and CPICH are spread in frequency domain bydifferent spreading codes, and then added and scrambled by thescrambling code C^((j))[k]. The scrambled signal is further combinedwith SCH signal, modulated via an N_(FFT)-point inverse discrete Fouriertransform (IDFT) (or, more efficiently, inverse fast Fourier transform(IFFT)), and inserted with GI to generate the downlink-transmittedsignal. The number of sub-carriers is exactly identical to the length ofthe scrambling code (K), and the IFFT size N_(FFT)≧K.

In the receiver of an MS, the received signal is processed by the cellsearch procedure shown in FIG. 2. The procedure involves three steps:(S1) symbol synchronization to detect OFDM symbol timing (OFDM symbolboundary), (S2) frame synchronization and group identification to detectflame timing (flame boundary) and the group code, and (S3)scrambling-code identification to detect the scrambling code C^((j))[k].In step S1, the symbol timing is detected by using the correlationproperty of CP. In step S2, after removing GI from the received signaland performing N_(FFT)-point discrete Fourier transform (DFT) (or, moreefficiently, fast Fourier transform (FFT)), the frame timing and groupcode are simultaneously detected by using SCH signal in frequencydomain. In step S3, the scrambling code C^((j))[k] is identified fromthe detected group by using CPICH signal, and verification is conductedto avoid false detection, thereby minimizing unnecessary MS activities.

Because SCH signal is not orthogonal to TCH signal and CPICH signal,cell detection performance of the SCH-based method is degraded due tothe interference from TCH signal and CPICH signal, and data detectionperformance is also degraded due to the interference from SCH signal.For this reason, the CPICH-based method (to be described next) does notinclude SCH signal into the frame structure, and thus performs muchbetter than the SCH-based method.

FIG. 3 shows the frame structure of the CPICH-based cell search method.Each frame consists of M OFDM symbols. Each OFDM symbol of lengthN_(OFDM) samples consists of N_(FFT)-sample useful data andN_(GI)-sample CP (GI). The first and last OFDM symbols, indicated byCPICH1 and CPICH2, respectively, correspond to CPICH signal, while theremaining (M−2) OFDM symbols are used for transmitting TCH data, whereR_(CPICH) is the power ratio of CPICH signal to the signal of one TCH.CPICH signal contains the information about the scrambling code, groupcode and frame timing. Because CPICH signal and TCH signal are allocatedin different OFDM symbols (i.e., different time slots), no interferencebetween them is incurred. Similar to the SCH-based method, the receiverof an MS for the CPICH-based method also uses the three-step cell searchprocedure shown in FIG. 2. The only difference is that in step S2, theframe timing and group code are simultaneously detected by using CPICHsignal, instead of SCH signal, in frequency domain.

Recall that step S1 for both the SCH-based and CPICH-based methods isperformed in time domain, while steps S2 and S3 in frequency domainusing N_(FFT)-point DFT (or FFT). In step S2, a lot of candidates fordetecting the frame boundary need to be tested in frequency domain tofind an optimum one. This implies that step S2 requires a lot of DFT (orFFT) operations for frame synchronization. Accordingly, the conventionalcell search methods require high computation complexity. Furthermore,cell detection performance of the CPICH-based method is sensitive tochannel effects because of a restrictive assumption for channel responsein step S3. When this assumption is not satisfied that is the typicalcase in practical application, it may lead to false detection.

In the European patent application EP0940942, a synchronization preambleand synchronization protocol for an MC-CDMA mobile communication systemis disclosed. The communication method enables remote stations tosynchronize in time and frequency to their serving base station. Itenables a base station and its remote stations in a cell to synchronizein a noisy environment where signal is interfered by other base stationsand remote stations in other cells. One major drawback of thecommunication method is the cell detection performance also sensitive tochannel effects.

SUMMARY OF THE INVENTION

The present invention has been made to overcome the above-mentionedshortcomings of conventional cell search methods for OFDM based cellularcommunication systems. An object of the present invention is to providea low-complexity and robust cell search method, whereby a new framestructure is introduced. For low-complexity cell search, the framestructure is designed to exhibit periodic signal pattern and meanwhilecontains the information about the cell code of desired cell in CPICHsignal. Furthermore, the cell search method utilizes the periodicproperty to detect frame timing, and the correlation property of CPICHsignal to identify the cell code.

According to the preferred embodiments of the invention, the cell searchmethod taking advantage of the property of periodic signal patternrequires only one or two DFT (or FFT) operations of small size, and isquite robust against channel effects. Simulation results demonstratethat the cell search method of the present invention outperforms theconventional CPICH-based method for both initial and target cell search.Better cell detection performance implies fewer iterations in cellsearch procedure for finding a cell-code candidate with high-confidencescore, and accordingly less average acquisition time as well as lowerpower consumption. As a consequence, the cell search method of thepresent invention provides the advantages of better link quality, fasteracquisition, and lower power consumption.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become better understood from a careful readingof the detailed description provided herein below with appropriatereference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention can be understood in more detail by reading thesubsequent detailed description in conjunction with the examples andreferences made to the accompanying drawings, wherein:

FIG. 1 shows the frame structure of the conventional SCH-based cellsearch method;

FIG. 2 is a flowchart showing the cell search procedure for conventionalcell search methods;

FIG. 3 shows the frame structure of the conventional CPICH-based cellsearch method;

FIG. 4 shows a time-domain frame structure according to the invention;

FIG. 5 is a flowchart showing a cell search procedure according to theinvention;

FIGS. 6 and 7 show other time-domain frame structures according to theinvention;

FIG. 8 a shows the time-domain frame structure according to the firstembodiment of the invention;

FIG. 8 b shows another view of the frame structure in FIG. 8 a;

FIG. 9 a shows the detailed structure of CPICH1 in FIG. 8 a;

FIG. 9 b shows the detailed structure of CPICH2 in FIG. 8 a;

FIG. 10 is a flowchart showing a cell search procedure according to thefirst and second embodiments of the invention;

FIG. 11 illustrates step 1001 in FIG. 10, in which the correlationproperty of CP in received signal is used;

FIG. 12 a shows the time-domain structure according to the secondembodiment of the invention;

FIG. 12 b shows the detailed structure of CPICH in FIG. 12 a;

FIG. 13 shows the number of complex multiplication operations requiredfor an iteration of cell search procedure versus FFT size for theconventional CPICH-based method and the first embodiment of the presentinvention; and

FIG. 14 a and FIG. 14 b plot the scrambling-code identificationperformance of desired cell versus geometry factor g for theconventional CPICH-based method and the first embodiment of the presentinvention for R_(CPICH)=6 dB and R_(CPICH)=9 dB, respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In an OFDM based cellular system, suppose that there are J cells in acluster and cell j is the desired cell to be searched for. The J cellsare differentiated by using J different cell codes, denoted byC^((i))[k], k=0˜L_(C)-1, i=1˜J, where L_(C) is the length of the cellcodes. The length L_(C) is chosen such that unique cell identificationin every cluster of J cells can be achieved, and it is not necessary tobe identical to the number of sub-carriers (K). To reduce the complexityof cell identification, every cell code can be further represented bytwo or more sequences. Without loss of generality, let a cell code berepresented by two sequences P^((i))[k], k=0˜L_(P)-1, iε{1, 2, . . . ,P-1}, and Q^((l))[k], k=0˜L_(Q)-1, lε{1, 2, . . . , Q-1}, where L_(P)and L_(Q) are the length of P^((i))[k] and Q^((l))[k], respectively, andP·Q≧J. Furthermore, let the cell code C^((j))[k] associated with cell jbe represented by the two sequences P^((p))[k] and Q^((q))[k]. Then,identification of the cell code C^((j))[k] is turn into the problem ofidentifying both the sequences P^((p))[k] and Q^((q))[k].

FIG. 4 shows a time-domain frame structure of the cell search methodaccording the present invention. Referring to FIG. 4, each frameconsists of M OFDM symbols, and each OFDM symbol of length N_(OFDM)samples consists of N_(FFT)-sample useful data and N_(GI)-sample CP(GI). The ith OFDM symbol, indicated by CPICHi, is comprised of CP andN_(i) repetitive duplicates of a v_(i)-point short sequence, whereN_(FFT)=v_(i)·N_(i) and N_(i)≧1. The other OFDM symbols in the frame mayinclude TCH signal or another CPICH signal. CPICH signal and TCH signalare allocated in different OFDM symbols (different time slots) to avoidinterference problem. In a frame as shown in FIG. 4, there is at leastone OFDM symbol (without considering GI) that exhibits periodic signalpattern. Also, there is at least one OFDM symbol in a frame thatcontains the information about the sequences P^((p))[k] and Q^((q))[k].

FIG. 5 shows the procedure of the cell search method according to theinvention. Referring to FIG. 5, the procedure includes two steps: (step501) timing synchronization to detect OFDM symbol timing and frametiming, and (step 502) cell code identification to detect the cell codeC^((j))[k]. Verification is included in step 502 to avoid falsedetection. In step 501, timing can be detected by using the correlationproperty of CP and the periodic property of the frame structure. In step502, the sequences P^((p))[k] and Q^((q))[k] are detected using thecorrelation property of CPICH signal.

FIGS. 6 and 7 show other time-domain frame structures of the cell searchmethod according the invention. Referring to FIGS. 6 and 7, no OFDMsymbol exhibits periodic signal pattern in a frame. Instead, in FIG. 6,there are at least two OFDM symbols in a frame that have the same datain some portions leading to periodic signal pattern in a frame. In FIG.7, CPICHi and CPICH(i+1) are viewed as a CPICH unit of length 2N_(OFDM)in which the length of CP is doubled, namely, 2N_(GI). For the framestructure in FIG. 7, there is at least a CPICH unit of length 2N_(OFDM)having periodic signal pattern in a frame. The periodic signal patternsin FIGS. 6 and 7 therefore can be used to detect the frame timing. Also,there is at least one OFDM symbol in a frame that contains theinformation about the sequences P^((p))[k] and Q^((q))[k] for cell codeidentification. Accordingly, the cell search procedure shown in FIG. 5can also be applied to the frame structures in FIGS. 6 and 7.

FIG. 8 a shows the time-domain flame structure according to the firstembodiment of the invention. Referring to FIG. 8 a, the first and secondOFDM symbols in a frame are CPICH1 and CPICH2, respectively. Theremaining (M−2) OFDM symbols in a flame, indicated by TCHs, are used fortransmitting TCH data. Clearly, CPICH signal is orthogonal to TCHsignal. Let s^((j))[n] denote the downlink-transmitted signal associatedwith cell j. In a flame as shown in FIG. 8 a, the transmitted signal ofmth OFDM symbol is represented as s_(m) ^((j))[n], n=0˜N_(FFT)-1,without taking GI into account. FIG. 8 b shows another view of the flamestructure in FIG. 8 a, where R_(CPICH) is the power ratio of CPICHsignal to the signal of one TCH and K is the number of sub-carriers usedin an OFDM symbol.

FIG. 9 a shows the detailed structure of CPICH1 in FIG. 8 a Thetime-domain CPICH1 signal, s₀ ^((j))[n], n=0˜N_(FFT)-1, is comprised ofN_(R) replica of the time-domain short sequence, x₁ ^((j))[n], n=0˜v-1,as follows:s ₀ ^((j)) [n]=x ₁ ^((j))[((n))_(v) ], n=0˜N _(FFT)-1   (1)where the notation ‘((n))_(v)’ denotes ‘(n modulo v)’. In other words,the time-domain CPICH1 signal s₀ ^((j))[n] is periodic with period v.The time-domain short sequence x₁ ^((j))[n] can be obtained by takingv-point IDFT (IFFT) of the frequency-domain short sequence, X₁^((j))[k], k=0˜u-1, where u·N_(R)=K and u≦v.

FIG. 9 b shows the detailed structure of CPICH2 in FIG. 8 a. Thetime-domain CPICH2 signal, s₁ ^((j))[n], n=0˜N_(FFT)-1, is comprised ofN_(R) replica of the time-domain short sequence, x₂ ^((j))[n], n=0˜v-1,as follows:s ₁ ^((j)) [n]=x ₂ ^((j))[((n))_(v) ], n=0˜N _(FFT)-1   (2)In other words, the time-domain CPICH2 signal s₁ ^((j))[n] is alsoperiodic with period v. The time-domain short sequence x₂ ^((j))[n] canbe obtained by taking v-point IDFT (IFFT) of the followingfrequency-domain short sequence:X ₂ ^((j)) [k]=X ₁ ^((j)) [k]·A ^((j)) [k], k=0˜u-1   (3)

The frequency-domain short sequences X₁ ^((j))[k] and X₂ ^((j))[k] (or,equivalently, X₁ ^((j))[k] and A^((j))[k]), k=0˜u-1, can be designed tocontain the information about the sequences P^((p))[k], k=0˜L_(P)-1, andQ^((q))[k], k=0˜L_(Q)-1. In particular, the sequence A^((j))[k] can bedesigned as follows:A ^((j)) [k]=P ^((p))[((k))_(L) _(P) ], for kεΩ ₁   (4a)andA ^((j)) [k]=Q ^((q))[((k))_(L) _(Q) ], for kεΩ ₂   (4b)where Ω₁ and Ω₂ are two disjoint sets of the indices of A^((j))[k], andΩ₁∪Ω₂⊂ {0, 1, . . . , u-1}. In this design, the cell search procedureshown in FIG. 5 can be implemented as the one shown in FIG. 10. Otherimplementations, however, are also feasible since the short sequence X₁^((j))[k] appeared in both CPICH1 and CPICH2 can be set arbitrarily. Forexample, the short sequence X₁ ^((j))[k] can be set to be identical forall cells, and thus it can be used as a training sequence forsimultaneous symbol and frame synchronization as well as for channelestimation.

Let the downlink-transmitted signal s^((j))[n] be received, after runits of time delay, by the receiver of an MS asr[n]=h ^((j)) [n]*s ^((j)) [n-τ]+w ^((j)) [n]  (5)where the notation ‘*’ denotes linear convolution operation. In (5),r[n] is the received signal, h^((j))[n] is the channel impulse response,and w^((j))[n] is the noise component including not only backgroundnoise but also interference from other cells and other MS's. The channelh^((j))[n] is assumed to be a linear time-invariant (LTI) finite impulseresponse (FIR) system of length L_(h). The unknown time delay τ can befurther expressed asτ=D·N _(OFDM) +d   (6)where D and d are non-negative integers, and 0≦d<N_(OFDM). According tothe cell search procedure shown in FIG. 10, the goal of the cell searchmethod of the embodiment is to estimate the unknown time delay d (OFDMsymbol timing) in step 1001, estimate the unknown time delay D (frametiming) in step 1002, and identify the sequences P^((p))[k] andQ^((q))[k] (the cell code C^((j))[k]) in step 1003.

In step 1001, the OFDM symbol timing d is detected by virtue of thecorrelation property of CP in the received signal r[n], as illustratedin FIG. 11. Correlations between the received signal and its delayedversion of N_(FFT) samples are computed and averaged as follows:$\begin{matrix}{{\Gamma_{1}\lbrack i\rbrack} = {\sum\limits_{m}{\sum\limits_{n = i}^{i + N_{GI} - 1}{{r\lbrack {{mN}_{OFDM} + n} \rbrack}{r^{*}\lbrack {{mN}_{OFDM} + n + N_{FFT}} \rbrack}}}}} & (7)\end{matrix}$where the superscript ‘*’ denotes complex conjugation. From (7), itfollows that the symbol timing d can be detected by finding the maximumof |Γ₁[i]|. Other symbol synchronization methods such as the well-knownmaximum-likelihood (ML) method and minimum mean-square-error (MMSE)method can also be applied in this step.

After the symbol timing d has been detected in step 1001, there stillremains an unknown time delay D·N_(OFDM) (see (6)) between thetransmitted signal s^((j))[n] and the received signal r[n+d]. In step1002, the frame timing D is detected by virtue of the property of theperiodic signal pattern in both CPICH1 and CPICH2. This is demonstratedfrom the following derivations.

By assuming the channel length L_(h)≦N_(GI)+1 (i.e., no ISI and ICI) andremoving GI from the received signal r[n] given by (5), the received mthOFDM symbol is obtained as: $\begin{matrix}{{{{r_{m}\lbrack n\rbrack} \equiv {r\lbrack {{mN}_{OFDM} + d + N_{GI} + n} \rbrack}}\quad = {{\sum\limits_{l = 0}^{L_{h} - 1}{{h^{(j)}\lbrack l\rbrack} \cdot {s_{m - D}^{(j)}\lbrack ( ( {n - l} ) )_{N_{FFT}} \rbrack}}} + {w_{m}^{(j)}\lbrack n\rbrack}}},{n = {{ 0 \sim N_{FFT}} - 1}}} & (8)\end{matrix}$where s_(m−D) ^((j))[n]≡s^((j))[(m−D)N_(OFDM)+N_(GI)+n] and w_(m)^((j))[n]≡w^((j))[mN_(OFDM)+d+N_(GI)+n] are the transmitted signal(without GI) and the noise associated with the received mth OFDM symbol,respectively. By ignoring noise effect, the received Dth OFDM symbolr_(D)[n] and (D+1)th OFDM symbol r_(D+1)[n], corresponding to CPICH1 andCPICH2, respectively, can be shown to be periodic with period v.Accordingly, the frame timing D can be detected by computing theautocorrelations of two successive received OFDM symbols as follows:$\begin{matrix}{{\Gamma_{2}\lbrack m\rbrack} = {\sum\limits_{i = 0}^{N_{R} - 1}{\sum\limits_{n = 0}^{v - 1}\{ {{r_{m}\lbrack {{vi} + n} \rbrack}{{r_{m}^{*}\lbrack ( ( {{v( {i + 1} )} + n} ) )_{N_{FFT}} \rbrack}\lbrack {{vi} + n} \rbrack}\quad{r_{m + 1}^{*}\lbrack ( ( {{v( {i + 1} )} + n} ) )_{N_{FFT}} \rbrack}} \}}}} & (9)\end{matrix}$The searching range of flame timing should involve at least (M+1) OFDMsymbols in (9) to ensure that two CPICHs, CPICH1 and CPICH2, areincluded in the computation. Since both r_(D)[n] and r_(D+1)[n],n=0˜N_(FFT)-1, are periodic with period v, Γ₂[m] given by (9) has amaximum value at m=D and, thus, the frame timing D is detected byfinding the maximum of |Γ₂[m]| given by (9).

After the frame timing D has been detected in step 1002, the cell codeC^((j))[k] is identified in step 1003 by identifying the associated twosequences P^((p))[k] and Q^((q))[k] by virtue of the frequency-domainrelationship between CPICH1 and CPICH2 given by (3), (4a) and (4b).

For reducing noise effect and computational complexity, the receivedOFDM symbol r_(D)[n] corresponding to CPICH1 is averaged over N_(R)replica to obtain $\begin{matrix}{{{{\overset{\_}{r}}_{D}\lbrack n\rbrack} = {\frac{1}{N_{R}}{\sum\limits_{i = 0}^{N_{R} - 1}{r_{D}\lbrack {{vi} + n} \rbrack}}}},{n = {{ 0 \sim v} - 1}}} & ( {10a} )\end{matrix}$In the same way, averaging the received OFDM symbol r_(D+1)[n] yields$\begin{matrix}{{{{\overset{\_}{r}}_{D + 1}\lbrack n\rbrack} = {\frac{1}{N_{R}}{\sum\limits_{i = 0}^{N_{R} - 1}{r_{D + 1}\lbrack {{vi} + n} \rbrack}}}},{n = {{ 0 \sim v} - 1}}} & ( {10a} )\end{matrix}$Taking v-point DFTs of {overscore (r)}_(D)[n] and {overscore(r)}_(D+1)[n], respectively, gives{overscore (R)} _(D) [k]={tilde over (H)} ^((j)) [k]X ₁ ^((j)) [k],k=0˜u-1   (11a)and{overscore (R)} _(D+1) [k]={tilde over (H)} ^((j)) [k]X ₁ ^((j)) [k]A^((j)) [k], k=0˜u-1   (11b)where {tilde over (H)}^((j))[k] is the v-point DFT of $\begin{matrix}{{h^{(j)}\lbrack n\rbrack} = \{ \begin{matrix}{{{h^{(j)}\lbrack n\rbrack} + {h^{(j)}\lbrack {n + v} \rbrack} + \ldots + {h^{(j)}\lbrack {n + {( {\lceil \frac{L_{h}}{v} \rceil - 1} )v}} \rbrack}},} & {n = {{ 0 \sim v} - 1}} \\{0,} & {otherwise}\end{matrix} } & (12)\end{matrix}$in which ┌a┐ denotes the smallest integer no less than a. From (12), itfollows that when L_(h)≦v, {tilde over (h)}^((j))[n]=h^((j))[n];otherwise, {tilde over (h)}^((j))[n] is an aliasing version ofh^((j))[n].

According to (11a), (11b) and (4a), the desired sequence P^((p))[k] canbe identified by computing $\begin{matrix}{{\Gamma_{3a}\lbrack i\rbrack} = {\sum\limits_{k \in \Omega_{1}}{{{\overset{\_}{R}}_{D}\lbrack k\rbrack}{{{\overset{\_}{R}}_{D + 1}^{*}\lbrack k\rbrack} \cdot {P^{(i)}\lbrack ( (k) )_{L_{p}} \rbrack}}}}} & (13)\end{matrix}$which has a maximum value for i=p. Thus, the desired sequence P^((p))[k]is identified by searching for the maximum of |Γ_(3a)[i]| given by (13)over P candidates of P^((p))[k]. In a similar way, according to (11a),(11b) and (4b), the desired sequence Q^((q))[k] can be identified bycomputing $\begin{matrix}{{\Gamma_{3b}\lbrack i\rbrack} = {\sum\limits_{k \in \Omega_{2}}{{{\overset{\_}{R}}_{D}\lbrack k\rbrack}{{{\overset{\_}{R}}_{D + 1}^{*}\lbrack k\rbrack} \cdot {Q^{(i)}\lbrack ( (k) )_{L_{Q}} \rbrack}}}}} & (14)\end{matrix}$which has a maximum value for i=q. As a result, the desired sequenceQ^((q))[k] is identified by searching for the maximum of |Γ_(3b)[i]|given by (14) over Q candidates of Q^((q))[k].

Unlike the conventional SCH-based and CPICH-based cell search methods,the process of finding the maximum of |Γ_(3a)[i]| given by (13) isindependent of that of |Γ_(3b)[i]| given by (14). When both thesequences P^((p))[k] and Q^((q))[k] are identified, the cell codeC^((j))[k] is correspondingly found.

The verification is included in step 1003 to avoid false detection. Let{circumflex over (p)} and {circumflex over (q)} be the indices obtainedby maximizing |Γ_(3a)[i]| and |Γ_(3b)[i]|, respectively. Then, theidentified cell code (i.e., the identified sequencesP^(({circumflex over (p)}))[k] and Q^(({circumflex over (q)}))[k]) canbe verified via the following ratios: $\begin{matrix}{\Delta_{3a} = {\frac{{\Gamma_{3a}\lbrack \hat{p} \rbrack}}{\max\{ {{{\Gamma_{3a}\lbrack i\rbrack}},{i \neq \hat{p}}} \}}\quad{and}}} & ( {15a} ) \\{\Delta_{3b} = \frac{{\Gamma_{3b}\lbrack \hat{q} \rbrack}}{\max\{ {{{\Gamma_{3b}\lbrack i\rbrack}},{i \neq \hat{q}}} \}}} & ( {15b} )\end{matrix}$When both Δ_(3a) and Δ_(3b) exceed a pre-assigned threshold, theidentified cell code is thought as a cell-code candidate with ahigh-confidence score. For this case, MS determines that the cell searchprocedure is completed successfully, and it then proceeds the subsequentprocesses such as frequency synchronization, read of broadcastinformation, and measurement of signal-to-interference-plus-noise ratio.Otherwise, MS continues the cell search procedure until a reliablecell-code candidate is obtained.

As shown above, the cell search procedure of the first embodimentrequires only two v-point DFT operations for step 1003. This impliesthat compared with the conventional SCH-based and CPICH-based methods,the computation complexity of the cell search method for the firstembodiment of the present invention is relatively low. Moreover, thecell search method for the first embodiment of the present inventionrequires no further assumption on the LTI FIR channel h^((j))[n],implying that it is quite robust against channel effects.

Subsequently, a second embodiment of the present invention withrelatively low complexity is provided.

FIG. 12 a shows the time-domain frame structure according to the secondembodiment of the present invention. Referring to FIG. 12 a, the firstOFDM symbol in a frame is CPICH. The remaining (M−1) OFDM symbols in aframe are used for transmitting TCH data FIG. 12 b shows the detailedstructure of CPICH in FIG. 12 a. The time-domain CPICH signal, s₀^((j))[n], n=0˜N_(FFT)-1, is comprised of N_(R) replica of thetime-domain short sequence, x^((j))[n], n=0˜v-1, as follows:s ₀ ^((j)) [n]=x ^((j))[((n))_(v) ], n=0˜N _(FFT)-1   (16)In other words, the time-domain CPICH signal s₀ ^((j))[n] is periodicwith period v. The time-domain short sequence x^((j))[n] can be obtainedby taking v-point IDFT (IFFT) of of the following frequency-domain shortsequence: $\begin{matrix}{{{X^{(j)}\lbrack {2k} \rbrack} = {B^{(j)}\lbrack k\rbrack}},{k = {{ 0 \sim\frac{u}{2}} - {1\quad{and}}}}} & ( {17a} ) \\{{{X^{(j)}\lbrack {{2k} + 1} \rbrack} = {{B^{(j)}\lbrack k\rbrack} \cdot {A^{(j)}\lbrack k\rbrack}}},{k = {{ 0 \sim\frac{u}{2}} - 1}}} & ( {17b} )\end{matrix}$

The frequency-domain short sequence X^((j))[k] (or, equivalently,B^((j))[k] and A^((j))[k]) can be designed to contain the informationabout the sequences P^((p))[k], k=0˜L_(P)-1, and Q^((j))[k],k=0˜L_(Q)-1. In particular, the sequence A^((j))[k] can be designed asfollows:A ^((j)) [k]=P ^((p))[((k))_(L) _(P) ], for kεΩ ₁   (18a)andA ^((j)) [k]=Q ^((q))[((k))_(L) _(Q) ], for kεΩ ₂   (18b)where Ω₁ and Ω₀₂ are two disjoint sets of the indices of A^((j))[k], andΩ₁∪Ω₂⊂ {0, 1, . . . , (u/2)- 1}. In this design, the sequence B^((j))[k]in (17a) and (17b) can be set arbitrarily.

Similar to the first embodiment of the present invention, the cellsearch procedure shown in FIG. 10 can also be used for the secondembodiment of the present invention. In step 1001, the OFDM symboltiming d is detected by finding the maximum of |Γ₁[i]| given by (7).After the symbol timing d has been detected in step 1001, the frametiming D can be detected by computing the autocorrelations of receivedOFDM symbols as follows: $\begin{matrix}{{\Gamma_{2}\lbrack m\rbrack} = {\sum\limits_{i = 0}^{N_{R} - 1}{\sum\limits_{n = 0}^{v - 1}{{r_{m}\lbrack {{vi} + n} \rbrack}{r_{m}^{*}\lbrack ( ( {{v( {i + 1} )} + n} ) )_{N_{FFT}} \rbrack}}}}} & (19)\end{matrix}$where r_(m)[n] is given by (8). Since the received Dth OFDM symbolr_(D)[n], corresponding to CPICH signal, is periodic with period v,Γ₂[m] given by (19) has a maximum value at m=D. Thus, in step 1002, theframe timing D is detected by finding the maximum of |₂[m]l given by(19).

After the frame timing D has been detected in step 1002, the cell codeC^((j))[k] is identified in step 1003 by identifying the associated twosequences P^((p))[k] and Q^((j))[k] by virtue of the frequency-domainrelationship given by (17a), (17b), (18a) and (18b). For reducing noiseeffect and computational complexity, the received OFDM symbol r_(D)[n],corresponding to CPICH, is averaged over N_(R) replica to obtain theaveraged time-domain signal {overscore (r)}_(D)[n] as given by (10a).Taking v-point DFT of {overscore (r)}_(D)[n] gives{overscore (R)} _(D) [k]={tilde over (H)} ^((j)) [k], k=0˜u-1   (20)where {tilde over (H)}^((j))[k] is the v-point DFT of {tilde over(h)}^((j))[n] given by (12).

According to (20), (17a), (17b), and (18a), the desired sequenceP^((p))[k] can be identified by computing $\begin{matrix}{{\Gamma_{3a}\lbrack i\rbrack} = {\sum\limits_{k \in \Omega_{1}}{{{\overset{\_}{R}}_{D}\lbrack {2k} \rbrack}{{{\overset{\_}{R}}_{D}^{*}\lbrack {{2k} + 1} \rbrack} \cdot {P^{(i)}\lbrack ( (k) )_{Lp} \rbrack}}}}} & (21)\end{matrix}$which has a maximum value for i=p as {tilde over (H)}^((j))[2k]≅{tildeover (H)}^((j))[2k+1]. Thus, the desired sequence P^((p))[k] isidentified by searching for the maximum of |Γ_(3a)[i]| given by (21)over P candidates of P^((p))[k]. In a similar way, according to (20),(17a), (17b), and (18b), the desired sequence Q^((q))[k] can beidentified by computing $\begin{matrix}{{\Gamma_{3b}\lbrack i\rbrack} = {\sum\limits_{k \in \Omega_{2}}{{{\overset{\_}{R}}_{D}\lbrack {2k} \rbrack}{{{\overset{\_}{R}}_{D}^{*}\lbrack {{2k} + 1} \rbrack} \cdot {Q^{(i)}\lbrack ( (k) )_{L_{Q}} \rbrack}}}}} & (22)\end{matrix}$which has a maximum value for i=q as {tilde over (H)}^((j))[2k]≅{tildeover (H)}^((j))[2k+1]. As a result, the desired sequence Q^((q))[k] isidentified by searching for the maximum of |Γ_(3b)[i]| given by (22)over Q candidates of Q^((q))[k]. When both the sequences P^((p))[k] andQ^((q))[k] are identified, the cell code C^((j))[k] is correspondinglyfound. Finally, the identified cell code is also verified as that in thefirst embodiment of the present invention.

As shown above, the cell search procedure for the second embodiment ofthe present invention requires only one v-point DFT operation for step1003, and thus its computation complexity is relatively low. Moreover,the cell search method for the second embodiment of the presentinvention requires only the channel assumption of {tilde over(H)}^((j))[2k]≅{tilde over (H)}^((j))[2k+1], which holds for typicalapplications. This therefore implies that the cell search method for thesecond embodiment of the present invention is robust against typicalchannel effects.

In the following, some calculation and simulation results regarding thefirst embodiment of the present invention are provided for verifying thepresent invention. An MC-CDMA cellular system was considered, in whichthe scrambling code (cell code) C^((j))[k], k=0˜K-1 (i.e., L_(C)=K), wasused and represented by only the sequence P^((p))[k], k=0˜u-1 (i.e.,L_(P)=u). In other words, the sequence Q^((q))[k], k=0˜L_(Q)-1, wasinexistent and the set Ω₂ in (4b) was accordingly an empty set for theillustration.

FIG. 13 shows the number of complex multiplication operations requiredfor an iteration of the cell search procedure versus FFT size for theconventional CPICH-based method and the first embodiment of the presentinvention. Less computation complexity means more power savings. FromFIG. 13, it can be seen that the computation complexity of theconventional CPICH-based method is about 4˜5 times higher than that ofthe first embodiment of the present invention.

FIG. 14 a and FIG. 14 b plot the performance of scrambling-codeidentification versus geometry factor, g, for the conventionalCPICH-based method and the first embodiment of the present invention forR_(CPICH)=6 dB and R_(CPICH)=9 dB, respectively. The geometry factor gof cell j is defined as $\begin{matrix}{g = \frac{E\{ {{{s_{0}^{(j)}\lbrack n\rbrack}}\quad}^{2} \}}{{E\{ {{w_{1}\lbrack n\rbrack}}^{2} \}} + {E\{ {{w_{2}\lbrack n\rbrack}}^{2} \}}}} & (23)\end{matrix}$where w₁[n] is the inter-cell interference and w₂[n] is the backgroundnoise. A high value of g indicates that MS is close to BS in cell j,whereas a low value of g indicates that MS is near cell boundary. FromFIGS. 14 a and 14 b, it can be seen that the cell search method of thepresent invention outperforms the conventional CPICH-based method forboth initial and target cell search, especially for the condition of lowvalue of g. This reveals that the cell search method of the presentinvention requires fewer iterations in the cell search procedure forfinding a cell-code candidate with a high-confidence score andaccordingly less average acquisition time as well as lower powerconsumption.

Although the present invention has been described with reference to thepreferred embodiments, it should be understood that the invention is notlimited to the details described thereof. Various substitutions andmodifications have been suggested in the foregoing description, andothers will occur to those of ordinary skill in the art For example, theframe structure may have a number of CPICHs other than one or two.CPICHs may be arranged in various OFDM symbols other than the first andsecond OFDM symbols in a frame. CPICHs may be periodic with differentperiods. Therefore, all such substitutions and modifications areintended to be embraced within the scope of the invention as defined inthe appended claims.

1. A method for synchronizing and identifying a cell code for anorthogonal frequency division multiplexing (OFDM) based cellularcommunication system, comprising the steps of: (a) building atime-domain frame structure for a cell search procedure, each frame insaid frame structure consisting of a plurality of OFDM symbols, saidframe structure exhibiting periodic signal pattern and containing theinformation about said cell code; and (b) performing said cell searchprocedure including the steps of timing synchronization and cell codeidentification.
 2. The method for synchronizing and identifying a cellcode for an OFDM based cellular communication system as claimed in claim1, wherein in step (b), said timing synchronization is to detect OFDMsymbol timing and frame timing, and said cell code identification is todetect said cell code.
 3. The method for synchronizing and identifying acell code for an OFDM based cellular communication system as claimed inclaim 1, wherein said cell search procedure in step (b) further includesa verification step to avoid false detection.
 4. The method forsynchronizing and identifying a cell code for an OFDM based cellularcommunication system as claimed in claim 1, wherein in a frame, there isat least one OFDM symbol that exhibits said periodic signal pattern andthere is at least one OFDM symbol that contains the information aboutsaid cell code.
 5. The method for synchronizing and identifying a cellcode for an OFDM based cellular communication system as claimed in claim1, wherein there are at least two OFDM symbols in a frame that have thesame data in some portions leading to said periodic signal pattern in aframe.
 6. The method for synchronizing and identifying a cell code foran OFDM based cellular communication system as claimed in claim 1,wherein there is at least one unit formed by two or more successive OFDMsymbols having said periodic signal pattern in a frame.
 7. The methodfor synchronizing and identifying a cell code for an OFDM based cellularcommunication system as claimed in claim 1, wherein at least one OFDMsymbol in a frame that contains the information about said cell code. 8.The method for synchronizing and identifying a cell code for an OFDMbased cellular communication system as claimed in claim 1, wherein eachOFDM symbol of length N_(OFDM) samples consists of N_(FFT)-sample usefuldata and N_(GI)-sample cyclic prefix (CP), the ith OFDM symbol,indicated by CPICHi, is comprised of CP and N_(i) repetitive duplicatesof a v_(i)-point short sequence, where N_(FFT)=v_(i)·N_(i) and N_(i)≧1,the other OFDM symbols in said frame includes traffic channel (TCH)signal or another common pilot channel (CPICH) signal, CPICH signal andTCH signal are allocated in different OFDM symbols.
 9. The method forsynchronizing and identifying a cell code for an OFDM based cellularcommunication system as claimed in claim 8, wherein said cell searchprocedure in step (b) uses the correlation property of CP and saidperiodic signal pattern of said frame structure to detect said timing.10. The method for synchronizing and identifying a cell code for an OFDMbased cellular communication system as claimed in claim 8, wherein saidcell search procedure in step (b) uses the correlation property of CPICHsignal to detect said cell code.
 11. A time-domain frame structure usedin cell detection for an orthogonal frequency division multiplexing(OFDM) based cellular communication system, said frame structureexhibiting periodic signal pattern to detect frame timing and containingthe information about the cell code of desired cell in common pilotchannel (CPICH) signal to identify said cell code.
 12. The time-domainframe structure used in cell detection for an OFDM based cellularcommunication system as claimed in claim 11, wherein each frame in saidframe structure consists of a plurality of OFDM symbols and each OFDMsymbol of length N_(OFDM) samples consists of N_(FFT)-sample useful dataand N_(GI)-sample cyclic prefix (CP), the ith OFDM symbol, indicated byCPICHi, is comprised of CP and N_(i) repetitive duplicates of av_(i)-point short sequence, where N_(FFT)=v_(i)·N_(i) and N_(i)≧1, theother OFDM symbols in said frame includes traffic channel (TCH) signalor another common pilot channel (CPICH) signal, CPICH signal and TCHsignal are allocated in different OFDM symbols.
 13. The time-domainframe structure used in cell detection for an OFDM based cellularcommunication system as claimed in claim 11, wherein said time-domainframe structure is introduced in a cell search procedure including thesteps of timing synchronization and cell code identification.
 14. Thetime-domain frame structure used in cell detection for an OFDM basedcellular communication system as claimed in claim 13, wherein said stepof timing synchronization is to detect OFDM symbol timing and frametiming, and said cell code identification is to detect said cell code.15. The time-domain frame structure used in cell detection for an OFDMbased cellular communication system as claimed in claim 11, wherein in aframe, there is at least one OFDM symbol that exhibits said periodicsignal pattern and there is at least one OFDM symbol that contains theinformation about said cell code.
 16. The frame structure used in celldetection for an OFDM based cellular communication system as claimed inclaim 11, wherein in a frame, there is at least one OFDM symbol thatexhibits said periodic signal pattern and there is at least one OFDMsymbol that contains the information about said cell code.
 17. The framestructure used in cell detection for an OFDM based cellularcommunication system as claimed in claim 11, wherein there are at leasttwo OFDM symbols in a frame that have the same data in some portionsleading to said periodic signal pattern in a frame.
 18. The framestructure used in cell detection for an OFDM based cellularcommunication system as claimed in claim 11, wherein there is at leastone unit formed by two or more successive OFDM symbols having saidperiodic signal pattern in a frame.
 19. The frame structure used in celldetection for an OFDM based cellular communication system as claimed inclaim 11, wherein at least one OFDM symbol in a frame that contains theinformation about said cell code.